This invention relates to an apparatus for manufacturing a semiconductor single crystal, and more particularly to an apparatus for manufacturing a semiconductor single crystal designed to effect growth of the semiconductor single crystal by pulling a fused mixture of raw materials for the semiconductor single crystal from a crucible heated by a resistance furnace while keeping a magnetic field applied to the crucible.
Among compounds of the elements of Families III-V, gallium arsenide (GaAs) exhibits especially outstanding electron mobility and is finding extensive utility as the crystalline substrate for elements in ultra-high speed integrated circuits and optoelectronic integrated circuits. GaAs is attracting keen attention because (1) when it is of high quality it acquires a high insulating property exceeding 10.sup.7 .OMEGA..cm in specific resistance, (2) it can be manufactured in a grade having minimal intracrystalline defects and enjoying even distribution of such defects, and (3) it can be easily manufactured in large wafers, for example. As a method capable of manufacturing a GaAs single crystal fulfilling all these requirements, the liquid-encapsulated Czochralski method (LEC method) is receiving increasing attention. This liquid-encapsulated Czochralski method is known to be available in either of the two versions; a low-pressure version and a high-pressure version. Since the low-pressure liquid-encapsulated Czochralski method uses as its starting material the GaAs polycrystal formed by the boat growth method, the single crystal suffers from low purity of raw material and requires incorporation of chromium (Cr), a substance capable of imparting a semi-insulating property to the crystal. The high-pressure liquid-encapsulated Czochralski method which effects synthesis directly on the starting material requires no addition of chromium. Since this method heats and synthesizes Ga and As, the raw material for crystal, and boron oxide (B.sub.2 O.sub.3), a liquid sealant, under high pressure, the fused mixture of raw materials for crystal in the crucible assumes a highly unstable state in the presence of thermal convection. Since this method effects the operation of crystal growth under such an unstable state, the shape of the solid-liquid boundary varies greatly and the growth crystal suffers from occurrence of fine striation and fine defects due to thermal variation and tends to entail dislocation and heterogeneous distribution of impurities. An integrated circuit of uniform electric property and device property cannot be easily manufactured with high repeatability using a crystal thus grown as its substrate since the defects so entailed in the crystal substrate cannot be controlled.
Concerning the manufacture of a single crystal of silicon (Si) or indium antimonide (InSb), the idea of effecting this manufacture by a procedure of pulling the crystal while under application of a magnetic field has been reported in literature. It has been found that when a magnetic field is applied to the aforementioned fused liquid of GaAs, generation of thermal convection within the fused liquid is repressed and the growth of crystal is consequently allowed to proceed with the solid-liquid boundary kept in a stable condition and, therefore, a GaAs single crystal of high quality can be obtained without entailing occurrence of striation.
Generally, the resistance furnace of the apparatus for the manufacture of such a single crystal is applied with and heated by alternating current or pulsating current. When the resistance furnace is exposed to a magnetic field, the magnetic force tends to exert a strong force on to the heater member of the furnace and cause damage thereto. To avoid this difficulty, Japanese Patent Application Disclosure Sho No. 56(1981)-45889 suggests use of direct current in the place of alternating current. Conversion of such a large amount of electric power as is consumed in the resistance furnace into a direct current is not easy. Further in most cases, the apparatus used for pulling a single crystal is designed to be operated with an AC power source. Thus, the use of direct current inevitably requires the apparatus to be remodeled to a great extent. Even when the apparatus is adapted to operate with a DC power source, the heater member of the furnace vibrates as the magnetic field to be applied increases beyond a certain level. As the vibration of the heater member is transmitted to the crucible, the effect of the application of the magnetic field is lost and the opposite effect is consequently induced. Thus, a semiconductor single crystal of high quality cannot be manufactured.
As described above, the convection of the fused mixture of raw materials for the crystal within the crucible can be repressed and a single crystal of high quality can be manufactured by applying a magnetic field to the neighborhood of the solid-liquid boundary of the fused mixture. Generally, the device used for the purpose of applying the magnetic field in the apparatus makes use of a pair of magnets disposed as fixed stationarily on the floor surface one each on diametrically opposite sides of the device used for pulling the crystal. To permit application of a powerful magnetic field, the magnets have a large volume and a heavy weight. They are fixed in an immovable state at the optimum positions selected relative to the crystal pulling device.
In the operation of pulling the crystal, it is necessary to (1) check the rotary driving unit of the crystal pulling device for possible effect of the application of magnetic field, (2) charge the device with the raw materials for the crystal, (3) mechanically adjust the device, (4) remove the formed crystal, and (5) clean the device. Particularly in the manufacture of GaAs single crystal, the multiplicity of carbon members used within the furnace must be cleaned after each batch operation. In the circumstances, there has been felt a pressing need for the development of an apparatus for the manufacture of a semiconductor single crystal, provided with a magnetic field applying device which concentrically applies a magnetic field near the solid-liquid boundary of the fused mixture of raw materials for the crystal, which permits easy handling, and which enables the various operations involved in crystal pulling to be carried out safely and quickly.